The subject matter described herein relates generally to methods and systems for operating a wind turbine, and more particularly, to methods and systems for operating a wind turbine below rated power. The subject matter furthermore relates to a controller for operating a wind turbine and a wind turbine.
Generally, a wind turbine includes a turbine that has a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators are sometimes, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbines also exist. The rotor, generator, gearbox and other components are typically mounted within a housing, or nacelle, that is positioned on top of a base that may be a truss or tubular tower.
Some wind turbine configurations include double-fed induction generators (DFIGs). Such configurations may also include power converters that are used to convert a frequency of generated electric power to a frequency substantially similar to a utility grid frequency. Moreover, such converters, in conjunction with the DFIG, also transmit electric power between the utility grid and the generator as well as transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid connection. Alternatively, some wind turbine configurations include, but are not limited to, alternative types of induction generators, permanent magnet (PM) synchronous generators and electrically-excited synchronous generators and switched reluctance generators. These alternative configurations may also include power converters that are used to convert the frequencies as described above and transmit electrical power between the utility grid and the generator.
Known wind turbines have a plurality of mechanical and electrical components. Each electrical and/or mechanical component may have independent or different operating limitations, such as current, voltage, power, and/or temperature limits, than other components. Moreover, known wind turbines typically are designed and/or assembled with predefined rated power limits.
A wind turbine can only extract a certain percentage of the power associated with the wind, up to the so-called maximum “Betz limit” of 59%. This fraction is described as the power coefficient. The value of the power coefficient is a function of the form, wind speed, rotation speed and pitch of the specific wind turbine. Assuming all other operational variables to be constant, this coefficient has only one maximum point at a fixed wind speed as the rotational speed is varied. It is therefore known to adjust the rotation speed of the turbine's rotor to this maximum value, that is called “optimal rotation speed” herein, in order to extract the maximum power possible out of the wind.
The characteristics of the power coefficient are normally expressed in terms of the tip-speed-ratio λ, which is defined as:
  λ  =                    v        p            v        =                  Ω        ·        R            v      wherein νp is the tip-speed of the one or more turbine blades, R is the turbine rotor radius, Ω is the rotational turbine angular velocity and ν is the wind speed.
In FIG. 3, the power coefficient CP is plotted as a function of the tip-speed ratio λ for an exemplary wind turbine. As it is evident from the diagram, the power coefficient is a function of the tip-speed ratio, and it has only one maximum value. Therefore, known wind turbines are operated at a rotation speed that corresponds to the tip-speed ratio which is called λmax in FIG. 3. Thereby it is possible to extract as much energy from the wind as theoretically possible.
In modern wind turbines, small improvements in energy yield may result in an essential increase of the return in investment of the turbines. It is thus an ongoing desire to further increase the annual energy production (AEP) of a wind turbine. The inventors of the present disclosure have found out a method to do so which is particularly applicable when the turbine is operated below rated power.